http: add on-upstream-connected access log flush (#26155)

Signed-off-by: Ohad Vano <ohadvano@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 758e277c3c436bf6e2acd285fa07a4d719065118

envoy/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto

@@ -36,7 +36,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // HTTP connection manager :ref:`configuration overview <config_http_conn_man>`.
 // [#extension: envoy.filters.network.http_connection_manager]
 
-// [#next-free-field: 55]
+// [#next-free-field: 56]
 message HttpConnectionManager {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager";
@@ -553,6 +553,11 @@ message HttpConnectionManager {
   google.protobuf.Duration access_log_flush_interval = 54
       [(validate.rules).duration = {gte {nanos: 1000000}}];
 
+  // If set to true, HCM will flush an access log once when a new HTTP request is received, after request
+  // headers have been evaluated, before iterating through the HTTP filter chain.
+  // Details related to upstream cluster, such as upstream host, will not be available for this log.
+  bool flush_access_log_on_new_request = 55;
+
   // If set to true, the connection manager will use the real remote address
   // of the client connection when determining internal versus external origin and manipulating
   // various headers. If set to false or absent, the connection manager will use the